The S&P 500 (SNPINDEX: ^GSPC) is an index of 500 companies listed on U.S. stock exchanges. It's a prestigious achievement for any company to be admitted into the index, and only the highest-quality names make the cut.

Selection is at the discretion of the Index Committee, but companies must be profitable, and they also need a market capitalization of at least $18 billion. That figure rises over time, because the S&P 500 is weighted by market cap, which means the largest companies in the index have a greater influence over its performance than the smallest.

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Source Fool.com